I do it too, put individual portions on a baking tray on grease proof paper and open freeze, then once frozen you can pop into freezer bags. I normally defrost in the morning and then reheat for a couple of minutes with a knob of butter.

I've only ever had disastrous results with freezing mash. It goes really watery, and heating it to steam off some of the excess liquid just turns it to a gluey wallpaper paste texture. Vile.

On the other hand, I've frozen potatoes parboiled ready for roasting in the past and these have always been amazing. (Parboiled, then shaken up inside the saucepan (with lid) with a spoon of plain flour, and separated out on a baking tray to go in the freezer. Tip them into a freezer bag once they're completely frozen. Roast them straight from the freezer.)

I don't know what they do to mash in industrial kitchens but ready made frozn mash can be absolutely excellent, whereas homemade frozen mash tends to retain too much water and go all woolly and unappealing.

I make a half potato half cauliflower mash and freeze it. The kids accept it either way, and it re-heats perfectly well. I don't add cream/butter, just boil in chicken stock, drain and mash - then when I reheat I add the cream etc. SO handy.